Ferrite stabilizing system



June 16, 1959 w. n. GABOR 2,891,158

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Gttornegs United States Patent FERRITE STABILIZING SYSTEM William D. Gabor, Norwalk, Conn., assignor `to C. G. S. Laboratories, Inc., Stamford, Conn., a corporation of Connecticut `Original application June 30, 1951, Serial No. 234,531,

now Patent No. 2,755,446, dated July 17, 1956. Divided and this application September '21, 1955, Serial No. 535,711

4 Claims. (Cl. Z50-36) The present invention relates to inductance control apparatus and to such apparatus in which a controlled inductance is used for measurement, communication, or control functions. The present invention is described as embodied in apparatus for regulating or controlling the inductance of one or a plurality of inductors having a core material of a ferromagnetic ceramic or ferrite This application is a division of copending application Serial No. 234,581, Ifiled June 30, 1951, now U.S. Patent No. 2,755,446.

It is now known in the art that certain alloys or ceramic materials known as ferrites or ferromagnetic ceramics have properties that make them promising as core materials for inductors. Some of the properties and specific compositions of these ferromagnetic ceramics are described, for example, in U.S. Patents 2,452,529; V2,452,- 530; and 2,452,531,

One of the properties of these ferromagnetic ceramics is that they have a relatively high permeability so that under proper conditions an inductor utilizing a small amount of this material as a core will exhibit a relatively high inductance. Another property of this material is that it has a relatively low saturation flux density, or in other words, its incremental permeability is greatly affected by small changes in the degree of magnetic saturation. lf such a core is subjected to a direct current magnetic field, sometimes referred to sas a bias field, the reactance of a coil wound on the core to the ow of a low-amplitude alternating current signal is decreased markedly.

By using inductors with this core material and by altering the magnetic condition of the core with a bias ield, it is possible to realize a variable inductance which can be varied over ranges as wide as to 1.

Another property Vof ferromagnetic ceramics is that they have relatively low loss characteristics, that is, the ratio between the energy stored per cycle by an inductor with this core material and the energy uselessly dissipated per cycle, is high over a wide range of operating conditions. This ratio, called the Q, is a measure of the quality of the inductor as an energy storage device. Thus, the Q of inductors utilizing ferromagnetic ceramic core material has been found to be high even when the core material is subjected to substantial magnetic fields and even whenthe inductor is used at frequencies of the order of megacycles.

Certain disadvantageous characteristics of ferromagnetic ceramics, however, has limited their use as core materials in inductors, particularly their sensitivity to changes in temperature. Moreover, the inductance of a coil wound on such material is a function of the past history of the core, so that the inductance at a particular time depends upon the previous state of core saturation. Also, changes in electrical loading affect their permeability and hence the inductance of a coil wound on such a core. In addition, it is difficult to replace such an inductor, for inductors using this core material have individualistic electrical properties so that two such in- Patented June 16, 1959 ductors which are of the same dimensions are likely to have different electrical and magnetic characteristics.

Accordingly, an `object of this invention is to provide apparatus for controlling or regulating the inductance of one or more inductors and by which the inductance may be stabilized and held at a predetermined value irrespective of changes in temperature or electrical load, and by which the hysteresis effects are overcome.

Another object of the present invention is to provide apparatus for precisely adjusting the inductance of one or more inductors having a ferromagnetic ceramic core material.

Another advantage of the invention resides in providing vcontrolled inductance with a relatively high Q for use in various electrical applications 'for measurement, control, communication, and similar functions.

Other objects relate to improved ferromagnetic ceramic components and combinations of components having use in measurement, control, and communication systems'as well as in other apparatus, and particularly to improved controlled and variable inductance components.

Other objects, advantages, and features of the present invention will be -in part apparent from, and in part pointed out in, the following description considered in conjunction with the accompanying drawings, in which:

Figure 1 is a diagrammatic rrepresentation of an inductance control circuit embodying the present invention;

Figure 2 is a diagrammatic representation of an inductance control circuit embodying the present invention arranged to control the inductance of a series of inductors in a delay line;

Figure 3 is a diagrammatic and schematic representation of a telemetering system embodying the present invention;

Figure 4 is a diagrammatic representation of a telenietering system embodying the present invention and using a permanent magnet to control the inductance of an inductor; and

Figure v5 is a perspective view of an inductor such as maybe u'sed'in the circuits of 'Figures 1 and 2 and, with certain changes, in the circuits of Figures 3 and 4.

In thevarious figures of the drawings similar parts or elements are identified by the same reference numeral followed by a small letter corresponding to the particular figure in which the part or-elemen't is shown.

Briey, the operation of the inductance control'circuit of Figure l is as follows: An inductor, generally indicated at 1t?, having a core material of ferromagnetic ceramic, is provided with an inductance-measuring winding 12 which is included in a bridge circuit `generally .indicated at 14, suitably energized by an oscillator 16. The output from the bridge circuit 14 is used to control an amplifier, indicated in block form at y17, and a phase detection system, generally indicated at 18, which provide a suitable control current `for a control winding 19 on the inductor 10.

This control current flowing through the winding 19 acts to maintain the inductance of inductor `10 at such value that the bridge 14 is in balance and therefore so that the output of the bridge14 is at a minimum. Thus, the inductance of inductor 10` is stabilized or controlled at a value corresponding to the setting of the bridge14, and an inductance or signal winding 20 on inductor it) can be 'used -to supply the controlled inductance for any desired 'external circuit.

The -bridge circuit 14includes a iirst arm comprising the measuring winding 12 connected in series with a resistor 24, a second arm comprising a variable resistor 28, a third arm comprising a resistor 34, and a fourth arm comprising theparallel combination of .a resistor 38 and condenser 42. Changes in the setting of the variable resistor 28 change the setting of the bridge 14 and hence change the inductance of inductor 10, as described hereinafter. t

In order to energize the bridge 14 with an A.C. voltage of suitable frequency, such as 1,000 c.p.s., the oscillator 16 has its output terminals 43 and 44 connected to the energizing terminals 45 and 46 of the bridge 14. The connection to the bridge input terminal 46 is made through the common ground circuit,

The inductance-measuring winding 12 on inductor 10 presents a reactance to the bridge 14 depending upon the permeability of the core 10, and because the permeability depends upon the strength of the bias magnetic eld, the winding 12 serves to present continuously to the bridge 14 a reactance which measures the magnetic condition of the core 10.

By selecting the values of the bridge element so that the product of the values of resistors 24 and 38 is equal to the ratio of the inductance of winding 12 to the capacity of condenser 42, the balance condition of the bridge can be made independent of frequency, the resistance of the winding 12 being included in the value assigned to resistor 24. Thus, at balance:

ai@ 1 R3. R28 lirjwRaeCo Rari-,751112 in L JBE l R l'bjwRssrz R24 l+jw1 R24 If n RaaRu-Oz then, at balance,

tazas Rza R24 and the balance condition is seen to be independent of frequency and to depend only upon the values of the circuit constants, and any tluctuations in the output of oscillator 16 will not affect the control of the inductor 10.

Whenever the inductance of winding 12 changes to unbalance the bridge 14, an alternating voltage of frequency corresponding to the frequency of the signal delivered by the oscillator 16, appears between bridge out put terminals 52 and 54. This voltage is connected from the bridge output terminals to the primary winding 57 of a transformer 58 having a grounded electrostatic shield 59. The secondary winding 60 of this transformer is connected to the input terminals 63 and 64 of the amplifier 17. The amplifier 17 may be of any design suitable for amplifying the alternating voltage.

The amplifier output terminal 67 is connected to the common ground circuit and the other output terminal 68 is connected 'through the primary winding 70 of an output transformer 72 to the positive terminal of a Conventional power supply 76, which is energized from power mains 78 through a main switch generally indicated at 82.

The secondary winding 84 of the output transformer' 72 is connected to the phase discriminating and rectify 111g circuit, generally indicated at 18.

In order to supply the phase-detection circuit 18 with a reference A.C. voltage from the oscillator 16, a lead 92 is connected from the terminal 43 on the oscillator 16 through a coupling condenser 94 to a control grid 9 5 of a cathode-follower tube 96. A ground return circuit is provided through a grid resistor 97 and a grid-bias battery 98. The anode 106 of this tube is connected directly to the positive terminal of the power supply 76. The cathode 110 of this tube is connected to the common ground circuit through a load resistor 112. A by-pass condenser 114 is connected between the anode 106 and ground. Y

The A.C. signal appearing across the cathode resistor 112 is coupled through the primary winding 116 of a transformer 118 to the secondary winding 122 and is applied between the common ground circuit and a centertap 123 on the secondary winding 84 of the transformer 72. The cathode-follower circuit drives the center tap 123 0f secondary 84 above and below ground potential in phase with the A.C. signal supplied by the oscillator 16, while isolating oscillator 16 from any load changes caused by the amplifier or phase detector.

The ends of secondary winding 84 of the transformer 72 are connected respectively to the plates 127 and 128 of a pair of diodes 132 and 134. The cathodes 136 and 138 of these tubes are connected to the common ground circuit through similar resistors 142 and 143, respectively,

The cathodes 136 and 138 are also connected to a lilter circuit, generally indicated at 143. The cathode 136 is connected to one plate of a iilter condenser 146, through a filter choke 148, to one plate of another lilter condenser 152 and then through a lead 154, to the control winding 19 on the inductor 10. The cathode 138 is connected to the other plate of the condenser 146, through a choke 157, to the other plate of the condenser 152, and then through a lead 158 and a source of bias current 159 to the other end of the winding 19.

The bias source 159 serves to provide a bias current flowing in winding 19 so as to establish a bias magnetic field in the ferromagnetic core material of the inductor 10. This bias current ows through a loop from the bias source 159 through winding 19, lead 154, choke 148, resistors 142 and 143, choke 157, and lead 158 to the other terminal of the bias source 159. The magnitude of this bias current is determined, among other things, by the voltage of the source 159 and the total resistance in the loop circuit described above. The voltage of bias source 159 should at least equal the voltage appearing across the condenser 146 when the bridge 14 is at its maximum unbalance caused by the inductor 10 having a smaller inductance than that corresponding to the setting of the variable resistor 28, thus the sum of bias and control current combined can never pass through zero and reverse direction, which would drive the bridge 14 away from balance.

The operation of the inductance control circuit of Figure l is as follows: Assuming that the bias field produced by the bias source 159 is not surTicient to produce an inductance correspondence to the setting of the variable resistor 28, then the inductance of inductor 10 will be too large and the bridge 14 will be unbalanced. The A.C. signal caused by the unbalance of the bridge 14 appears across the output terminals 52 and 54 and is amplified by the amplifier 17 and is fed to the phase detection and rectifier circuit 18 through the output transI former 72. This A.C. signal drives the plates 127 and 128 of diodes 132 and 135, respectively, in opposite directions, and during each half cycle the polarity reverses so that as one of these plates is driven positive, the other plate is driven negative. At the same time, an A.C. signal from the oscillator 16 is fed through the cathode follower tube to the centertap 123 of the secondary winding 84 driving it alternately positive and negative. The polarity of the voltage at the centertap 123 may be in phase with the voltage either at the plate 127 or thc plate 128, depending upon the direction of the unbalance of bridge 14, that is, upon whether the inductance of the inductor 10 is larger or smaller than the value correspond ing to balance. The circuit connections are such that when the bias source 159 produces a bias field in inductor 10 causing winding 12 to exhibit too large an inductance, then the output from the phase detection and rectifier circuit 18 aids the bias source 159 so as to reinforce the magnetic bias tield and hence reduce the inductance of the inductor 10 to the desired value, i.e. corresponding to the setting of variable resistor 28.

When the bias eld is such that winding 12 has too little inductance, the bridge 14 is unbalanced in the opposite direction, causing a decrease in bias current through the winding 12.

Assuming that the voltage at centertap 123 is in phase with the voltage at plate `127, caused by the voltage induced in the transformer secondary winding 84, these two voltages aid to produce a large positive voltage at the plate 127, in turn causing a large current to ilow through diode 132 and from cathode 136 through the resistor 142 to ground. This current causes a positive voltage to appear at the cathode 136. Since the plate 128 of diode 134 is negative, no current flows through the diode 134 nor through the resistor 143 except for the current from the bias source 159 and hence the other terminal of the condenser 146 is near ground potential, and the condenser 146 is charged by the diode 132.

When theplate 128 is driven positive during the next half cycle by the voltage induced in the transformer secondary winding 84, the voltage at the centertap 123 is being driven negative by the cathode-follower tube 95. These two voltages are opposed, resulting in only a small positive voltage on the plate 128. Consequently, only a small current flows from the cathode 138 through the resistor 143 to ground. Thus, only a small positive voltage appears at the cathode 138 and only a small portion of the charge on the condenser 146 is removed during the half-cycle when diode 134 is conducting. The time constant of the loop circuit including the resistors 142 and 143 and the condenser preferably is suciently large so that very little of the charge on condenser 146 leaks off through resistor 142 during each cycle. ln this explanation, it was assumed that both diodes conducted during alternate half-cycles, but it is apparent, of course, that under different degrees of unbalance of the bridge 14, only one of the diodes may conduct, the voltage of centertap 12,3 serving to suppress conduction during alternate half-cycles.

Thus, it is seen that this circuit automatically adjusts the saturation level `of Ithe core material to maintain the inductance of winding 12 as a known predetermined function of the value of variable resistor 2S and that the controlled condition is independent of changes in temperature and the like. Because the winding 20 is wound on this controlled core, its inductance is controlled precisely. Furthermore, this circuit allows continuous control or regulation of the -inductance of winding 20 at any desired value over a yrange of 20C' to l, which can be obtained with ferromagnetic ceramic cores, for readjustment of the variable resistor 28 immediately causes a corresponding readjustment in the inductance of the winding 20. If, for any reason, it is necessary to replace the inductor 10, the replacement will be controlled at the same value of inductance.

Although a particular control circuit is described herein, it is apparent that many differentcontrol circuits can be used. The use of the filter chokes 148 and 157 and of the filter condenser 152 is optional, for they merely filter the D.C. current supplied to the control winding 18, which may only be Vdesirable when the bias field in ythe inductor is to be maintained with substantially no ripple. Also, it -should be noted that although the windings 12 and 19 on the inductor 10 are shown as two separate windings, a single winding may be used to perform the functions of both, as explained hereinafter. f

Furthermore, `instead of the use ofthe bias source 159, it is possible to associate a permanent magnet with the inductor 10 to establish the bias eld. Another way of obtaining the bias eld in inductor 10 is to energize the measuring winding 12 with a DC. current in addition to I In Figure 2 is shown a diagrammatic representation of an induetance control circuit for controlling the inductance of a number of similar inductors, generally indicated at 10a, 164, 166, and 168 in an electrical delay line, generally indicated at 172.

Each of these inductors 16a, 164, 166 and 163 has an inductance or signal winding 20a, 174, 176, and 178, respectively, and these windings are serially connected in the delay line 172 between input terminal 132 and output terminal 184. The delay line 172 also includes a number of condensers 186, 13S, 192, 194, and 196 connected in conventional manner across the delay line. As is Well known in the art, such a delay line may be used to delay the transmission of electrical impulses passing therethrough in either direction, and the delay time is a function of the inductance of the windings 20a, 174, 176, and 17S.

in order to control the inductance of the inductors 10a, 161i-, 166 and 168 so that the delay time of the line 172 can be varied at will and with precision, and in order to stabilize each of them so that environmental changes, such as temperature, or load, etc. will have no effect upon the inductance of these inductors and hence no effect upon the delay time of the line 172, a control arrangement similar to Figure 1 is provided. The oscillator 16a, the bridge, generally indicated at 14a, and the amplifier and phase detection system 17a correspond to the components of Figure l, the principal diiference being that the control windings 198, 266, and 202 of inductors 164, 166, and 166 are connected in series with the control winding 19a of inductor 10a, the latter being the only inductor that is provided with a measuring winding 12a which is connected into the bridge circuit 14a.

T he control circuits operate as described in connection with Figure 1 to control the inductance of winding 20a of inductor 10a. Because the inductors have the same characteristics and are subjected to the same ambient and operating conditions, the same control current is utilized te control each of the inductors. Thus, the circuit of Figure 2 stabilizes the inductance of each of the inductors ma, 164 and 166 and 168 and hence stabilizes the delay time of the line 172.

In order to vary the delay time of line 172, it is only necessary to vary the resistance of the variable resistor 23a. For instance, a motor can be connected to drive the variable resistor over a cycle of values of resistance, and the delay time of line 172 would cyclically follow these values, or the resistor 28a can be manually controlled, to regulate the delay time of line 172. The resistance 23a may be replaced by electronic control means, if desired, so that the delay line can be adapted readily for use in radar, control, measuring, and communication systems.

1n Figure 3 is shown a diagrammatic representation of a telemetering system embodying the present invention. In this embodiment, a variable inductor 260, having a core of ferromagnetic ceramic, is arranged with its inductance winding 261 connected in parallel with a condenser 262 to form a tuned circuit, of a Hartley-type oscillator, generally indicated at 266.

Brieiiy, the operation of this telemetering circuit is as follows: The frequency of the oscillator 266 is determined by the inductauce of the winding 261 of the variable inductor 260, and the output signal of the oscillator is transmitted over a pair of long distance leads 268 and 269, or by other means, to a discriminator 270 which produces an output signal whose magnitude is a function ef the frequency of the receiver signal. This output current, for example, may operate a meter 271, which in effect indicates the inductance value of the remotely located inductor winding 261.

T he inductance of this winding 261 in turn is controlled by the value of a variable resistor 2819, which can be made responsive to a condition which is being measured. Thus, meter 271 indicates the value of the resistor ZSbif and hence indicates the status of the condition under measurement.

In the oscillator circuit, the inductance winding 261 is provided with a centertap 276, connected by a lead 278 to the grounded cathode 282 of a beam power tube 284. The control grid 286 of this tube is connected through a grid-leak resistor' 288 and grid condenser 292 to one end of the winding 261. The screen grid 296 of this tube is connected through a blocking condenser 298 to the other end of winding 261. As is well known in the art, with this type of connection, the tube 204 acts as a triode oscillator, with the screen-grid 296 acting as the anode in the oscillator circuit. Its plate 302 is coupled to the oscil lator circuit by means of the electrons passing through the screen-grid 296, and the plate load resistor 304 de velops an A.C. voltage at the resonant frequency of the tuned circuit.

This A.C. voltage across resistor 304- is connected to the distance discriminator 270 by the long distance leads 268 and 269. Conventional power supply means, indicated diagrammatically by the batteries 312 and 314 are provided.

The connections of oscillator` 16h and bridge 1411 are not described in detail, for they are similar to the circuits of Figures l and 2. The inductance measuring winding 315 is connected into the bridge circuit 14D through a pair of coupling condensers 316 and 317. The coupling condensers 316 and 317 are sufficiently large so that their reactance is negligible at the frequency supplied by the oscillator 16h, and hence the mathematical analysis made in connection with the bridge 14 in Figure l is valid for bridge 14b. Thus, the balance point of bridge 14]) is independent of variations in the frequency of oscillator 16b. It should be noted that by using the coupling condensers 316 and 317 the winding 315 is made to serve both the measuring function for bridge 14h and also the controlling function. Consequently, inductor 260 need carry only two windings. The filter chokes 322 and 323 should have inductances much larger than the inductance of winding 315.

ln operation, the variable resistor 2811 is arranged to vary in accordance with a condition under measurement. These variations in the resistor 23h cause the inductancc of the winding 261 to vary correspondingly, and this causes the frequency of the oscillator 266 to vary, thus changing the indication of the meter 271.

In Figure 4 is shown a diagrammatic representation of a telemetering system embodying the present invention and using a movable permanent magnet 330 to control the inductance of an inductor 332 having a core of ferromagnetic ceramic material. The inductor has a mearuring winding 334 included in a bridge circuit 14C which is identical with the bridge 14h of Figure 3 except that the variable resistance h in Figure 3 is replaced in Figure 4 by a diagrammatic representation of a unit at 335 containing a resistive condition under measurement.

The oscillator 16e energizes bridge 14C, and the output from the bridge Mc is fed to an amplifier and phase detection system 17C. The output signal from `system 17c drives a reversible DC. motor 344. This motor 344 drives a worm and gear, generally indicated at 346, that operates a rack 352 to position the permanent magnet 330. When the bridge 14C is unbalanced in direction, the motor 344 drives the permanent magnet 330 nearer to the inductor 332, thus increasing the magnetic bias in the core thereof and reducing its in ductance, and drives the magnet 330 away from the inductor 332 when the bridge is unbalanced in the opposite direction.

Thus, the inductance of the inductor 332 is stabilized continuously by the action of the oscillator l6c, bridge 14C, and the amplifier and phase detection system lc. Furthermore, the inductance of inductor 332 is controlled by the resistance value of unit 335. An inductance or signal winding 356 of the inductor 332 in series with a condenser 35S forms a series resonant circuit whichy is connected by two long-distance leads 362 and 363 to an oscillator and discriminator, shown in vblock form at 366, of a type shown in the copending patent application of Carl G. Sontheimer, Serial No. 65,094, filed December 14, 1948, now U.S. Patent No. 2,621,517.

Thus, as the resistive condition in unit 335 changes, the inductance of winding 356 changes and alters the resonant frequency of the series resonant circuit. The frequency of the oscillator and discrirninator 366 is changed, resulting in a different output of the meter 374. Thus, meter 374 continuously shows the condition of remotely located unit 336, and this reading will be independent of variations in shunt capacity between the leads 364 and 363 as described in any earlier application.

A preferred form of variable inductor, generally indicated at 400 in Figure 5, has an annular core 401 composed of a ferromagnetic ceramic material, one portion of which is slotted as shown at 403.

A measuring winding 404 is wound around an unslotted portion of the core 401, and a bias and control winding 406 is wound around another un-slotted portion of the core as shown.

An inductance or signal winding 408 is divided into two halves, and each half is wound around one-half of the cross-section of the slotted core as shown. In order to minimize the coupling between both the measuring winding 404 and the bias and control winding 406 and the signal winding 408, the two halves of the signal winding 408 are wound in opposite directions, so that any voltages induced in the winding 408 caused by changes in the bias field of winding 406 or the eld of winding 404 will oppose each other and cancel out. Likewise, the magnetic field caused by the signal winding 408 will be limited substantially to the slotted portion of the core 401 and will not interfere with the measuring winding 404.

Alternatively, the interaction between the measuring Winding 404 and the signal winding 408 can be minimized by forming the measuring winding 404 of two equal and oppositely wound portions while rusing a continuously wound coil for winding 408.

What is claimed is:

l. Apparatus for producing a variable frequency signal as a function of resistance variation comprising an inductor having a ferrite core, first and second windings on said core, a balanceable electrical network including manually variable resistance means and capacitive means coupled to said first winding, an alternating current source connected to said network, a variable frequency oscillator having a tuned circuit including said second winding and a capacitor coupled to said second winding, means responsive to unbalance of said network for producing a direct control current to said rst winding to modify the magnetic saturation of said core in such direction as to tend to maintain said network in balanced condition, whereby the frequency of said oscillator can be controlled by said variable resistance means and is stabilized against changes in temperature and other external conditions to which said core is sensitive.

2. Apparatus for producing a variable frequency signal as a function of resistance variation comprising an inductor having a core of ferromagnetic material, first and second windings coupled to said core, a balanceable electrical network including variable resistance means and said first winding, an alternating current source connected to said network, a variable frequency oscillator having a tuned circuit including said second winding and a capacitor coupled to said second winding, and means responsive to imbalance of said network for modifying the magnetic saturation of lsaid core in such direction as to tend to maintain said network in balanced condition, whereby the frequency of said oscillator can be controlled by said variable resistance means and is stabilized against changes in temperature and other external conditions to which said core is sensitive.

3. Apparatus for producing a variable frequency signal as a function of resistance variation comprising an inductor having a core of ferromagnetic material, first and second windings coupled to said core, a balanceable electrical network including variable resistance means and said rst winding, an alternating current source connected to said network, a variable frequency oscillator having a tuned circuit including said second winding and a capacitor coupled to said second winding, and means including phase-detecting means responsive to unbalance of said network for modifying the magnetic saturation of said core in such direction as to tend to maintain said network in balanced condition, whereby the frequency of said oscillator can be controlled by said variable resistance means and is stabilized against changes in temperature and other external conditions to which said core is sensitive.

4. Apparatus for producing a variable lfrequency signal as a function of resistance variation comprising an indnctor having a magnetically saturable core, rst, second and third windings on said core, a balanceable electrical network including variable resistance means and l() capacitive means coupled to said iirst winding, an alternating current source connected to said network, a variable requency oscillator having a tuned circuit including said second winding and capacitance means coupled to said second winding, means responsive to unbalance of said network for producing a direct control current to said rst winding to modify the magnetic saturation of said core in such direction as to tend to maintain said network in balanced condition, whereby the frequency of said oscillator can be controlled by said variable resistance means and is stabilized against changes in temperature and other external conditions to which said core is sensitive, and an external circuit connected to said third winding and responsive to the magnetic condition of said core for controlling said external circuit.

References Cited in the le of this patent UNITED STATES PATENTS 2,483,314 DeGroot Sept. 27, 1949 2,495,634 Hepp Ian. 24, 1950 2,544,643 Ahrendt et al. Mar. 13, 1951 2,600,342 er lune 10, 1952 

